Inkjet printing systems are also are well known in the art. Small droplets of liquid ink, propelled by thermal heating, piezoelectric actuators, or some other mechanism, are deposited by a printhead on a print media, such as paper.
In scanning-carriage inkjet printing systems, inkjet printheads are typically mounted on a carriage that is moved back and forth across the print media. As the printheads are moved across the print media, the printheads are activated to deposit or eject ink droplets onto the print media to form text and images. The print media is generally held substantially stationary while the printheads complete a “print swath”, typically an inch or less in height; the print media is then advanced between print swaths. The need to complete numerous carriage passes back and forth across a page has meant that inkjet printers have typically been significantly slower than some other forms of printers, such as laser printers, which can essentially produce a page-wide image.
The ink ejection mechanisms of inkjet printheads are typically manufactured in a manner similar to the manufacture of semiconductor integrated circuits. The print swath for a printhead is thus typically limited by the difficulty in producing very large semiconductor chips or “die”. Consequently, to produce printheads with wider print swaths, other approaches are used, such as configuring multiple printhead dies in a printhead module, such as a “page wide array”. Print swaths spanning an entire page width, or a substantial portion of a page width, can allow inkjet printers to compete with laser printers in print speed.
One type of inkjet printing system utilizes multiple printhead modules that each print a substantial portion of a page width. The printhead modules in this type of system may include multiple printhead die linearly spaced across the print swath, such that each die prints a portion of the swath, typically one inch or less. Correcting for die-to-die variation in a printing system may be desirable, as well as correcting for overall color accuracy of the printer. One common method of color correction involves measuring printed test samples using a calibrated color sensor.
A problem with color calibration is to get accurate color measurements. When a built-in sensor is incorporated into the printer, the problem is made more difficult by cost constraints and memory requirements. A built-in sensor is typically a lower-cost device without the full capabilities of a spectrophotometer; typically, such devices may be non-linear with respect to a colorimetric color space, and may exhibit significant device-to-device variation. Limited memory in a printing system may also make storing large amounts of calibration data impracticable.
There is thus a need for apparatus and methods for calibrating color sensors in printing systems.